1. Field of the Invention
The invention relates to the processing of epitaxial layers upon crystalline substrates, and more particularly to an apparatus for growing epitaxial layers by means of a metal organic molecular beam.
2. Prior Art
The Metal Organic Molecular Beam Epitaxy ("MOMBE") technique is used to grow epitaxial layers upon a semiconductor substrate in the formation of semiconductor devices. The growth takes place upon a heated semiconductor substrate in a growth chamber into which one or more reagents in a gaseous or vapor state are introduced.
The MOMBE reagents are converted to a gaseous or vapor state in a bubbler by passing a carrier gas through a metal organic reagent in liquid form. These reagents are suitable for epitaxial formation and are capable of saturating the small flow of carrier gas as it passes through the bubbler. The carrier borne gaseous MO compounds are then injected into the growth chamber where they decompose into "metallic" and organic components upon encountering the heated substrate. Ideally, the organic component escapes and is removed by the vacuum pumps while the "metallic" component bonds to another "metallic" component on the heated substrate to form the epitaxial layer.
The letter "M" in the acronyms "MOMBE" and "MO", accordingly stands for "metal" but includes elements from Groups II to VI. Group III metals, which bond with Group V semi-metals, form the III-V semiconductor compounds which are more common. Among the III-V compounds are gallium arsenide, aluminum gallium arsenide, gallium indium arsenide, indium phosphide, gallium phosphide, gallium indium phosphide, gallium indium arsenide phosphide, and indium antimonide.
Epitaxial processing, when properly carried out, produces crystalline layers which have uniform lattices, accurate impurity distributions, and accurately gauged thicknesses.
A plurality of differing layers may be required in common semiconductor devices. For instance, in a high electron mobility transistor (HEMT), the final structure consists of five discrete layers, each optimized to enhance transistor performance. The layers include the substrate, which is 20 mils thick, which may be of indium phosphide. The first epitaxially formed layer is also of indium phosphide, and is one micron thick. The first epitaxial layer is followed by one 800 .ANG. layer of gallium indium arsenide, a 400 .ANG. layer of aluminum indium arsenide with a 45 .ANG. undoped underportion, and a final 200 .ANG. layer of gallium indium arsenide.
The performance of the HETT and devices optimized for high frequency performance depends on accuracy-ideally to within an atomic layer - (2-3 .ANG.) in the uniformity of the thickness of each of the several epitaxial layers. In addition it is desired that the transitions between layers - the hetero-interfaces - be abrupt. The present apparatus is intended to provide means for forming layers of this quality and multiple layer structures with abrupt hetero-interfaces.
The MOMBE technique incorporates the key advantages of two prior technologies; MOVPE (metal-organic vapor-phase-epitaxy) and MBE (molecular-beam-epitaxy).
MOVPE reactors generally provide good control and reproducibility of the molar flow of metal organic reagents. However, hydrodynamic processes such as gas-phase depletion, convection and turbulence occur in MOVPE reactors where the growth chamber pressures are typically from 0.1 to unity atmospheric pressure. At these pressures, the flow of the injected gases is viscous and can become turbulent, which limits the accuracy, uniformity and reproducibility of the epitaxial layers and of devices fabricated from these layers.
In MBE, hydrodynamic problems are eliminated by use of a vacuum (molecular flow regime) environment in the growth chamber using solid sources. However the uniformity, reproducibility and throughput are unfavorably affected by depletion effects in conventional solid sources.
MOMBE combines the accurately metered and long lived MO gas sources of MOVPE with the vacuum environment of MBE and has the potential for a higher uniformity, reproducibility and thoughput than either predecessor.
Apparatus optimized to carry out the MOMBE process is accordingly necessary for efficient MOMBE processing. Central to such apparatus is the manifold for delivery of MO reagents to the MOMBE growth chamber.
In particular, the manifold in MOMBE processing must maintain reasonably high MO molar flows with modest total gas flows. In MOMBE, the pressure in the growth chamber must remain below 10E-4 torr in order to maintain the molecular flow regime essential to uniformity i epitaxial layer formation. If excessive carrier gas flows are used, then the pumping speeds required to evacuate the reactor chamber to the required low pressure become prohibitive.
The maximum permissible total gas flow into the MOMBE chamber is approximately 50 standard cubic centimeters per minute (sccm), assuming currently available pumping speeds. In an MOVPE system, total gas flows of 10 standard liters/min (slm) (200 times greater than in MOMBE) are common and carrier flows in each bubbler are typically 50 to 100 sccm. When MO reagents having a low vapor pressure are used in MOVPE, the bubbler flow may be 400 sccm or even higher if an acceptable epitaxial growth rate is to be achieved. It is not possible to use such high bubbler flows in MOMBE processing. Accordingly, if low vapor pressure materials are to be used in MOMBE, the process conditions must be altered from the MOVPE processing conditions.
In short, in MOMBE processing, a manifold is required, which will inject adequate molar flow rates of the MO reagent, while using low carrier flow rates. In addition, the manifold should permit accurate metering of the MO molar reagents, and should accommodate reagents having low vapor pressures.